These things need not be eternal or ancient. For all we know, these conduits are created by "snowplowing" them with a transwarp coil, and then either kept open with help from a transwarp hub, or left to deteriorate and collapse.
It may also be that one needs to establish a conduit at both ends, meaning one has to fly to the far end at conventional warp speeds first. Which is what the Borg have only done twice before "Endgame"; perhaps it took two tries? Or a third, unfilmed one?
What Janeway did in "Endgame" is far from clear. Perhaps she collapsed the entire network, perhaps she just hurt that one hub. Only the younger Janeway believes in "dealing a crippling blow"; Admiral Janeway merely speaks of blowing up the local hub and getting the crew home.
According to dialogue there were six transwarp hubs in the galaxy, while the graphic only showed four hubs, one in each quadrant. Additionally, according to dialogue, there were only exit apertures in the Alpha Quadrant, while the graphic shows a hub in the Alpha Quadrant as well.
The dialogue doesn't actually tell us that those four dots on screen would be hubs. At that point of the show-and-tell, the heroes merely discuss the exit points in all the four quadrants; those four could be examples of such, just before a more complex schematic of the corridor network is displayed.
What part of the dialogue should be taken to mean there aren't hubs in the Alpha Quadrant?